CNG Tank Design

A Freshman Engineering Design Project
Developed by Dr. Joey K. Parker, The University of Alabama

  Background Information

Natural gas is becoming an increasingly attractive fuel for many transportation uses. Fuel costs are significantly less than with gasoline, although the cost of the conversion kits and required accessories are not insignificant. Abundant reserves of natural gas are available in the U.S., which reduces energy dependence on foreign suppliers. Most importantly, significantly lower emissions are possible due to the simple chemical structure of natural gas (primarily methane - CH₄). Natural gas composition varies considerably over time and from location to location. Methane content is typically 85-95% with the remainder primarily ethane, propane, and carbon dioxide. Much of the natural gas produced from coal seams in Tuscaloosa county is quite high in methane content, often over 95%. The combustion characteristics of methane are similar enough that unmodified gasoline engines may be successfully operated using natural gas.

The primary disadvantage of natural gas as a transportation fuel is that under standard conditions it exists as a gas, and thus has a low density. At atmospheric pressures and temperatures, a "gallon" (231 in³ or 0.00379 m³) of natural gas at standard atmospheric conditions contains about 1.4x10⁵ joules of energy, compared to 1.3x10⁸ joules for a gallon of gasoline. Since the volumetric energy density (joules/m³) is so low, natural gas is often stored in a compressed state at high pressure.

 Large steel or composite wrapped aluminum pressure vessels (similar to those used with torch welding systems) store the compressed natural gas (CNG) at pressures of 3000 psi (2.07x10⁷ Pa). Tanks are readily available in a variety of lengths and diameters, as shown in the figure below.

 Steel tanks are significantly cheaper (material and fabrication costs) than the aluminum/composite wound tanks that are sometimes used. However, the weight penalty for steel tanks may be prohibitive in load-sensitive applications (tractor-trailer trucks for example). A relatively new entry into the CNG tank market is the composite reinforced steel tank shown below

Note the shape of the tanks shown above. Pressure vessels are almost exclusively spherical or cylindrical in shape due to fabrication ease as well as strength. A "box" type construction would be difficult to seal (welds on the edges) and of lower strength per unit volume and per unit weight. Cylindrical tanks are easily fabricated (rolled or extruded seamless tube) with hemispherical end-caps welded on. The hemispherical end-caps represent a major portion of the cost for cylindrical cylinders.  Spherical tanks are not space and size efficient in trunks or other potential locations for automobiles or light trucks.

 Regardless of the shape and construction material used for the tank, the ideal gas law can be used to estimate the mass of natural gas stored at any given pressure once the volume and temperature are known.

©2002, Joey K. Parker
These materials may be duplicated for educational purposes if properly credited.